In recent years, the development of continuous reaction processes has aggressively proceeded using microchannels composed of microtubes having a cross-sectional area of about 0.01 cm2 or less. This technology is characterized by improving reaction controllability by improving the efficiency of heat removal by taking advantage of the high specific surface area of these microchannels, and improving reaction efficiency by increasing the efficiency of mass transfer.
Known examples of being able to significantly improve reaction controllability in a continuous reaction process include a direct fluorination reaction using fluorine for the gas phase, and using microchannels to efficiently control reactions between hydrogen and oxygen (Non-Patent Document 1).
With respect to reactions within a microreactor composed of microchannels, an increase in the interface area within the microchannels promotes mass transfer and improves reaction efficiency, while an increase in specific surface area of the reactor facilitates temperature control, and these reactions are characterized by being able to achieve reaction conditions and reaction selectivity unable to be achieved with conventional reactors. Consequently, the prior art consists nearly entirely of proposals relating to microreactors, or the shape and size of microchannels, while there have been few disclosures regarding optimum reaction conditions or catalyst usage environment in these reactors.
A production process of epoxy compounds characterized by epoxidating olefin compounds from hydrogen peroxide has been proposed as an example of a production process carried out in a microreactor using a catalyst (Patent Document 1). In this document as well, there are no particular limitations on the oxidation catalyst used and a known oxidation catalyst can be used, and there are also no particular limitations on the liquid distributing method used when supplying the mixture of olefin compound and hydrogen peroxide to the microreactor, and a known method can also be employed. However, the catalyst particle diameter disclosed in the examples is 1 micron or less, and considerable pressure loss is predicted to occur within the microreactor. It is clear that special considerations are required to be given to the liquid distribution method from the viewpoint of stable operation of the reaction.
A process for carrying out hydrogenation of aldehyde compounds or nitro compounds under mild conditions by catalytic hydrogenation is proposed as an example of a production process carried out in a microreactor using a catalyst (Patent Document 2). In this document, although the examples of hydrogenation catalysts used in the microreactor include a palladium catalyst, nickel catalyst, platinum catalyst and ruthenium catalyst, the mean particle diameter of the hydrogenation catalyst is normally preferably about 0.1 to 100 μm and particularly preferably about 1 to 50 μm, and the ratio of catalyst mean particle diameter to flow path diameter is preferably about 0.1 or less and particularly preferably about 0.07 or less, there is no disclosure regarding the hydrogen flow rate and efficacy of the use of hydrogen is an issue from the viewpoint of reaction efficiency.
A technology has been proposed for producing water and at least one type of alkene and/or aralkene by allowing a hydrocarbon-containing fluid containing an alkane or aralkane and an oxygen source into microchannels having a catalyst therein and by allowing the hydrocarbon-containing fluid and the oxygen source to react within the microchannels within a temperature range of 300 to 1000° C. (Patent Document 3). In this document as well, there are no particular limitations on the catalyst active substance used, and any conventional effective oxidative dehydrogenation catalyst can be contained, while there is no particular disclosure relating to the catalyst usage environment for enhancing, reaction efficiency. In addition, the target reaction of this technology is a gas phase reaction, and it is not clear whether this technology can be applied as it is to a mixed gas/liquid phase reaction.
Next, reactions of hydrogen and oxygen are expected to be applied to a hydrogen peroxide production process. In the past, hydrogen peroxide has been produced by a reaction process referred to as the anthraquinone process. In this production process, however, the anthraquinone successively decomposes during the operation of the process, and has been indicated as having problems such as the formation of impurities resulting from decomposition products thereof contaminating the hydrogen peroxide as the final product, and as a result thereof, studies have been conducted for many years for replacing this process with a direct reaction process using hydrogen and oxygen that does not result in the formation of impurities.
At present, the use of microreactor technology to a hydrogenation step to impart higher productivity than a conventional hydrogenation reactor based on the anthraquinone process has been disclosed as an example of a commercially implemented hydrogen peroxide production process (Patent Document 4). In this document as well, the catalyst used in the hydrogenation reactor is indicated having any size or geometrical shape that is compatible for the use within microchannels, and the hydrogenation catalyst may be alternatively filled into the microreactor channels or deposited, dispersed or coated onto a conventional catalyst support introduced therein. In this document as well, there is no particular disclosure relating to the catalyst usage environment for enhancing reaction efficiency. In addition, the present technology does not solve the basic problems of anthraquinone process of successive decomposition of the anthraquinone and resulting contamination of the hydrogen peroxide as the final product.
In the production of hydrogen peroxide by a direct reaction process using hydrogen and oxygen, studies have been previously conducted on a reaction system composed of an aqueous solution containing a trace amount of a stabilizer for stably recovering hydrogen peroxide and a catalyst composed mainly of a precious metal such as palladium or gold. More specifically, hydrogen peroxide is formed by allowing hydrogen and oxygen dissolved in water to react on a catalyst (Non-Patent Document 2).
This direct reaction process has several problems from the viewpoint of safety and productivity when attempting to make it industrially available. First, since the hydrogen and oxygen form an explosive mixed gas over an extremely wide range, it was necessary to operate under conditions in which hydrogen partial pressure was reduced to 4% or less in the prior art. In addition, since hydrogen peroxide is formed by reaction of hydrogen and oxygen dissolved in water, it was necessary to improve the dissolution efficiency of each component.
In order to solve the problems of this direct reaction process, a continuous process technology using microchannels is considered to be effective. For example, the inventors of the present invention developed a microreactor by fabricating microchannels on silicon, and constructed a microreactor in which a supported palladium catalyst was filled into the microchannels of the microreactor.
This microreactor was effective for producing hydrogen peroxide both safely and stably despite operating under explosive conditions in which the hydrogen content is 20 to 50% when producing hydrogen peroxide from a mixed gas of hydrogen and oxygen. In addition, quantitative evaluation of mass transfer from the gas phase to the liquid phase revealed that mass transfer is 10 to 100 times more efficient than conventional reactors.
The reason why an explosive composition of a mixed gas of hydrogen and oxygen can be handled safely by this microreactor is that propagation of the explosion is prevented by the use of microchannels, and improvement of efficiency of mass transfer is thought to be the result of having increased the contact interface between gas and liquid due to filling the microchannels with a catalyst having a small particle diameter.
However, the concentration of the resulting hydrogen peroxide was held to a low value of 0.2% by weight, this was considered to be caused by a problem with introducing the gas and liquid phases into the microreactor based on the results of experiments consisting of visualizing gas flow (Non-Patent Document 3).
On the other hand, Van den Bussche et al. have disclosed a process for producing hydrogen peroxide following the production of hydrogen and oxygen by electrolysis with respect to a hydrogen peroxide production process based on microchannels (Patent Documents 5 to 8). However, both the details of the reactor and the concentration of hydrogen peroxide are unclear.
In addition, Tonkovich et al. have provided a detailed disclosure of a reactor structure with respect to a hydrogen peroxide production process (Patent Document 9). However, this document does not contain a detailed disclosure of the reaction conditions, and the performance of the reactor is completely unclear.
Moreover, Lawal et al. have proposed a reactor in which a palladium-supported catalyst is filled into SUS pipes having an inner diameter of 775 μm, and disclosed the synthesis of hydrogen peroxide having a maximum concentration of 1.1% by weight (Patent Document 10). However, in this reactor, a large excess gas of hydrogen and oxygen are required to flow through the reactor in order to form a stable flow of the mixed gas/liquid phase, thereby resulting in the problem of having to recycle the gases unreacted.
Now, in developing a continuous reactor that uses microchannels, it is essential to arrange several to more than ten rows of microchannels in parallel, for example, in order to ensure a required production volume while ensuring high reaction controllability. In this case, in order to realize productivity as expected, it is necessary to make the reaction conditions uniform for each microchannel.
For example, in the hydrogen peroxide production process developed by the inventors of the present invention (Non-Patent Document 3), although ten rows of microchannels are arranged in parallel, variations in the flow of the mixed gas/liquid phase to each microchannel were observed by visually analyzing the flow. This was thought to be the reason for the loss of productivity in the hydrogen peroxide production process disclosed in Non-Patent Document 3.
On the other hand, Kitamori et al. have disclosed a parallel arrangement method by laminating glass reactors (Patent Document 11). However, while to this method is limited to liquid phase reactions, it also has the problems, with increasing the degree of parallel operation of an increase in the possibility of different flow rates of the reaction solution for each microchannel due to drift which causes a decrease in the productivity of the reactor.
In addition, Tonkovich et al. have developed a reactor in which microchannels are arranged in parallel, and have shown that a fluid can be evenly distributed to each microchannel (Patent Document 12). However, this document only discloses the case of distributing the flow of a single type of fluid for this reactor, and whether or not it can be applied to a mixed gas/liquid phase reaction as in the case of producing hydrogen peroxide is not clear.
Moreover, Wada et al. have developed a reactor for carrying out an ozone oxidation reaction in which 16 microchannels are arranged in parallel, and have shown that in this reactor, post-shape structures are cumulatively fabricated within each microchannel using microfabrication technology, and that a mixed gas/liquid phase flow is formed with an improved mass transfer efficiency (Non-Patent Document 4). However, this reactor has problems from the viewpoint of integrating a catalyst within the microchannels when considering application to a solid catalyst reaction, and it is not clear whether or not an even gas-liquid mixed phase flow is formed among the 16 microchannels under conditions in which a catalyst has been integrated.
In this manner, in the case of technologies relating to conventional microreactors composed of microchannels, since it is difficult for the mixed gas/liquid phase reaction to accommodate industrial production, the development of a continuous reactor using microchannels that is able to accommodate industrial production and allow continuous reaction has been a strongly favored in the technical field.
Patent Document 1: Japanese Patent Application Laid-open No. 2007-230908
Patent Document 2: Japanese Patent Application Laid-open No. 2006-248972
Patent Document 3: PCT/US2003/016210 (WO/2003/106386)
Patent Document 4: PCT/US2006/033851 (WO/2007/027767)
Patent Document 5: U.S. Pat. No. 6,713,036
Patent Document 6: U.S. Pat. No. 7,115,192
Patent Document 7: U.S. Pat. No. 7,192,562
Patent Document 8: U.S. Pat. No. 7,195,747
Patent Document 9: U.S. Pat. No. 7,029,647
Patent Document 10: U.S. Patent Publication No. 2006/0233695A1
Patent Document 11: Japanese Patent Application Laid-open No. 2002-292275
Patent Document 12: U.S. Patent Publication No. 2007/0246106A1
Non-Patent Document 1: Volker Hessel, Steffen Hardt and Holger Loewe co-authors, “Chemical Micro Process Engineering-Fundamentals, Modeling and Reactions”, 2004, publisher: Wiley-VCH Verlag GmbH & Co., KGaA, Weinhelm (ISBN: 3-527-30741-9)
Non-Patent Document 2: Jose M. Campos-Martin et al., “Hydrogen Peroxide Synthesis: An Outlook Beyond the Anthraquinone Process”, Angewandte Chemie International Edition, Vol. 45, 6962-6984 (2006)
Non-Patent Document 3: Tomoya Inoue et al., “Microfabricated Multiphase Reactors for the Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen”, Industrial and Engineering Chemistry Research, Vol. 46, 1153-1160 (2007)
Non-Patent Document 4: Yasuhiro Wada et al., “Flow Distribution and Ozonolysis in Gas-Liquid Multichannel Microreactors”, Industrial and Engineering Chemistry Research, Vol. 45, 8036-8042 (2006)